As a story idea for Kotor that keeps it canon and Believable, it should be set after the Revan Novel. However, since kotor is based on Revan and his experiences, the best place for Kotor 3 to be set would be the Mandalorian Wars. It ties into the story of Revan, is kept canon and we can see characters from the games, not to mention learning the backstories of Kreia/Traya, Sion and Nihilus who were severely lacking in story. Thats the only place for kotor to go.